Electron lens



Nov. 1, 1949 G. LXEBMANN 2,486,856

ELECTRON LENS Filed Oct. 14, 1948 IYWVTWtOPz 6er-har-d Ljlelmmann HiAttovn ey.

Patentecl Nov. 1, 1949 ELECTRON LENS Gerhard Liebmann,

tion of New York Cambridge, England, assignor to General ElectricCompany, a corpora- Application October 14, 1948, Serial No. 54,381 InGreat Britain April 12, 1947 10 Claims. 1

This invention relates to improvements in lenses of the magnetic type,such as are employed for focusing a beam of electrons.

Lenses employing magnetic or electric fields to focus beams of particlessuch as electrons have been used extensively in recent years. Inparticular, these lenses have found useful application in an instrumentcommonly known as an electron microscope, such as is described in anarticle entitled Electron microscopy appearing in the General ElectricReview, December 1944.

lhe resolving power of the electron microscope is limited by thediffraction eifect and by spherical and chromatic aberration in theelectron lenses, particularly in the objective lens, even if spacecharge effects, asymmetries leading to astigmatism, and other defects ofthe electron optical system are avoided. The image of a point situatedon the electron-optical. axis is spread out into a disc of confusion,the radius of which, when referred back to the object space, isapproximately given by to, to spherical aberration, 6s, and to chromaticaberration, c, are given by In these equations, a is the semi-angle ofthe imaging electron bundle, ll the electron wave length, V theaccelerating voltage, AV the fluctuation of the accelerating voltage, Hthe magnetic field strength, and AH the fluctuation of the magneticfield strength.

The values of the constants in and R22 are at present not quite certain,values of between 0.5 and 1.0 being usually assumed. The sphericalaberration constant Cs and the two chromatic aberration constants Covand Con have, in previously constructed instruments, values of 0.2 cm.to 2.0 cm. It has also recently been shown that the magnetic saturationof iron pole pieces forming the electron lenses leads to an optimumresolving power for an accelerating voltage of V=50 to 60 kv. \-5x10cm.), the resolving power of the best present instruments approachingThe present invention provides a means of correcting primary sphericalaberration in a magnetic electron lens which comprises combining withsaid lens a diverging electrostatic electron lens that has a marginalfocal length which is smaller than the paranial focal length. Such alens can, for instance, be realized by applying an electrostatic fieldbetween a conducting plane or curved metal film, which is thin enough'to be permeable to electrons, and a cylinder spaced a short distanceaway from the metal film and having its axis perpendicular to it, theelectric field vector pointing from the metal film toward the cylinder.

A diverging lens of this kind has been found to have the requiredproperty of a marginal focal length which is shorter than the paraxialfocal length 0. The relative difference o of the marginal or zonal focallength of the described diverging lens can be varied over a wider rangeby varying the diameter or distance of the said cylinder and byadjusting the potential difference between metal film and cylinder. Itis possible to produce a considerable amount of longitudinal zonalspherical aberration even in weak lenses, and the strong convergingmagnetic lens need not lose much of its refractive power by beingcombined with a correcting diverging electrostatic lens. As the same lawgoverns primary aberration in weak and in strong lenses, and in positiveand negative lenses, it is possible to correct spherical aberration overthe whole range governed only by primary spherical aberration. Thus, alens can be corrected in the described way up to semi-angles of a-0.2,whereas the uncorrected electron microscope objectives used hitherto canonly accept rays of a semi-angle of less than 0.01 before sphericalaberration limits the resolving power.

Other objects and advantages of the invention will appear from thefollowing detailed description taken in conjunction with theaccompanying drawing which is a diagrammatic cross section of anelectron lens suitably embodying the invention.

The electron lens shown includes the combination of an electromagneticlens and an electrostatic lens. The electromagnetic lens comprises anannular exciting coil l which is enclosed by an envelope 2, preferablyof metallic material such as iron, that is provided in its bore with aspaced apart pair of pole pieces 3 and 4. The envelope 2 may be extendedalong its bore, as shown, to provide a means for making the lens anintegral part of. an electron miroscope. A spacer 5 of non-magneticmaterial such as brass is inserted within a gap in the bore of theenvelope 2 between the pole pieces and may be suitably sealed therein toform a vacuum-tight enclosure for the exciting coil. To provideexcitation for the exciting coil, leads H and 12 may be brought outthrough the envelope 2 in a suitable manner and connected to a source ofdirect current potential (not shown). The electrostatic lens comprisestwo electrodes, one of which includes a metal film 6 which may bemounted as shown on a supporting ring I, secured to the pole piece 3 sothat it extends across the bore through the pole piece. The otherelectrode is formed by pole piece 4 which may be insulated from theenvelope 2 by a sleeve 8 and connected through an insulated lead 9 to asuitable source of direct current potential (not shown).

The thickness of the metal film 6 should be such that the image qualityis not impaired by electrons being scattered to an appreciable extent,but should obviously be as great as is con sistent with this requirementfor reasons of mechanical strength and accuracy. Thus, for lowerelectron accelerating voltages, it may be necessary to employ very thinbut conducting metal films produced by evaporating aluminum or berylliumonto a very thin plastic film. At voltages in excess of 100 kv., itshould be possible to employ thin metal films stretched tightly over asuitable frame. Although the film is illustrated as being substantiallyplanar, it is to be under stood that it may be curved in a convenientmanner, the only requirement being that the portion through whichelectrons pass be normal to the optical axis. Also, the film 6 may beconstructed in the form of a grid to minimize scattering of the electronbeam.

Although the insulation of the pole piece 4 from the envelope 2introduces a second air gap into the magnetic circuit of the lens, theeffect of this second air gap need not be appreciable if, as ispracticable, the reluctance of the gap between pole piece 4 and theenvelope 2 is made considerably lower than the reluctance of the gapbetween the two pole pieces 3 and G.

The bores through the pole pieces 3 and 4 are generally cylindrical incross section, However, it may be desirable for some applications toconstruct the pole pieces with slightly elliptical bores or to providemeans for distorting the bores slightly from a cylindrical shape.Therefore, the term cylinder-like will be employed as descriptive of theshape of the pole piece bores, and it will be understood that this termincludes a slight departure from a truly cylindrical shape.

The invention has been described in connection with an electronmicroscope lens system in order that practical adaptations of thefeatures of the invention may be adequately illustrated. It is obvious,however, that the above-described features can be applied to lensesother than electron microscope objective and projector lenses; forexample, to lenses in high Voltage or cathode ray tubes or lenses infine X-ray tubes and the like.

Having described the principle of this invention and the best mode inwhich I have contemplated applying that principle, I wish it to beunderstood that the apparatus described is illustrative only, and thatother means can be employed without departing from the true scope of theinvention expressed in the following claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electron lens comprising a spaced apart pair of pole pieces havingcylinder-like bores coaxially aligned, a winding about both said polepieces, means for exciting said winding to provide a magnetic fluxlinking said pole pieces to form a magnetic lens, a conductive electronpermeable member extending normal to the axis of said pole pieces withinthe space separating the adjacent ends of said pole pieces, andconnecting means for establishing a potential difference between saidmember and at least one of said pole pieces to provide an electrostaticdiverging lens having a marginal focal length smaller than its paraxialfocal length.

2. An electron lens comprising a spaced apart pair of pole pieces havingcylinder-like bores coaxially aligned, a winding about both said polepieces, means for exciting said winding to provide a manetic fluxlinking said pole pieces to form a magnetic lens, an electron permeablemember extending across the bore of one of said pole pieces at the endnearest the other of said pole pieces, and connecting means forestablishin a potential different between said member and said otherpole piece to provide an electrostatic diverging lens having a marginalfocal length smaller than its paraxial focal length.

3. An electron lens comprising a first hollow cylinder-like pole piece,a, second hollow cylinderlike pole piece coaxial with and spaced fromsaid first pole piece, an annular winding about both said pole pieces,means for exciting said winding to provide a magnetic flux linking saidpole pieces whereby a magnetic lens is produced, a thin electronpermeable member supported by said first pole piece within the spaceseparating the adjacent ends of said pole pieces, and connecting meansfor establishing a potential difference between said member and saidsecond pole piece to provide an electrostatic diverging lens having amarginal focal length smaller than its paraxial focal length.

4. An electron lens comprising a spaced apart pair of pole pieces havingcylinder-like bores coaxially aligned, a winding about both said polepieces, means for exciting said winding to provide a magnetic fluxlinking said pole pieces to form a magnetic lens, a thin electronpermeable member conductively attached to one of said pole pieces at theend nearest the other of said pole pieces, and connecting means forestablishing a potential difference between said member and said otherpole piece to provide an electrostatic diverging lens having a marginalfocal length smaller than its paraxial focal length.

5. An electron lens comprising a spaced apart pair of pole pieces havingcylinder-like bores coaxially aligned, a winding about both said polepieces, means for exciting said winding to provide a magnetic fiuxlinking said pole pieces to form a magnetic lens, a thin electronpermeable member conductively attached to and extending across the boreof one of said pole pieces at the end nearest the other of said polepieces, and connecting means for establishing a potential differencebetween said member and said other pole piece to provide anelectrostatic diverging lens having a marginal focal length smaller thanits paraxial focal length.

6. An electron lens comprising a spaced apart pair of pole pieces havingcylinder-like bores coaxially aligned; an annular winding enclosed by ametallic envelope disposed about both said pole pieces, one of said polepieces being insulated from said envelope; means for exciting saidwinding to provide a magnetic flux linking said pole pieces to form amagnetic lens; an electron permeable member conductively attached to thenon-insulated pole piece at the end nearest the insulated pole piece,and insulated means for connecting a source of potential to saidinsulated pole piece to provide a potential difference between saidmember and said insulated pole piece to form an electrostatic diverginglens having a marginal focal length smaller than its paraxial focallength.

'7. An electron lens comprising a spaced apart pair of pole pieceshaving cylinder-like bores coaxially aligned; an annular Windingenclosed by an envelope disposed about both said pole pieces, saidenvelope having an annular non-magnetic portion in its bore coextensivewith the space between the adjacent ends of said pole pieces; means forexciting said winding to provide a magnetic flux linking said polepieces to form a magnetic lens; an electron permeable member extendingnormal to the axis of said pole piece bores and within the spaceseparating the adjacent ends of said pole pieces, and connecting meansfor establishing a potential difference between said member and saidother pole piece to provide an electrostatic diverging lens having amarginal focal length smaller than its paraxial focal length.

8. An electron lens comprising a spaced apart pair of pole pieces havingcylinder-like bores 00- axially aligned; an annular winding enclosed bya metallic envelope disposed about both said pole pieces, said envelopebeing insulated from one of said pole pieces and having an annularnonmagnetic portion in its bore coextensive with the space between theadjacent ends of said pole pieces; means for exciting said winding toprovide a magnetic flux linking said pole pieces to form a magneticlens; an electron permeable member conductively attached to thenon-insulated pole piece at the end adjacent the insulated pole piece,and insulated means for connecting a source of potentia1 to saidinsulated pole piece to provide a potential difference between saidmember and said insulated pole piece to form an electrostatic diverginglens having a.

marginal focal length smaller than its paraxial focal length.

9. An electron lens comprising a spaced apart pair of pole pieces havingcylinder-like bores 00- axially aligned and tapered adjacent endportions; a winding enclosed by a metallic envelope disposed about bothsaid pole pieces, said envelope being insulated from one of said polepieces and having an annular non-magnetic portion in its borecoextensive with the space between said pole pieces defined by theadjacent non-tapered portions thereof; means for exciting said windingto provide a magnetic flux linking said pole pieces to form a magneticlens; an electron permeable member conductively attached to thenon-insulated pole piece at the end adjacent the insulated pole piece,and insulated means for connecting source of potential to said insulatedpole piece to provide a potential difference between said member andsaid insulated pole piece to form an electrostatic diverging lens havinga marginal focal length smaller than its paraxial focal length.

10. An electric lens comprising a spaced apart pair of pole pieceshaving cylinder-like bores coaxially aligned; a winding enclosed by ametallic envelope disposed about both said pole pieces, said envelopebeing insulated from at least one of said pole pieces; means forexciting said winding to provide a magnetic flux linking said polepieces to form a magnetic lens; an electron permeable member extendingnormal to the axis of said pole piece bores and within the spaceseparating the adjacent ends of said pole pieces, and insulated meansfor connecting a source of direct current between said member and atleast one of said pole pieces to provide an electrostatic diverging lenshaving a marginal focal length smaller than its paraxial focal length.

GERHARD LIEBMANN.

No references cited.

Certificate of Correction Patent No. 2,486,856 November 1, 1949 GERHARDLIEBMANN It is hereby certified that errors appear in the printedspecification of the above numbered patent requiring correction asfollows:

Column 1, line 50, for 10 read 10 cm.; column 2, line 1, for paranialread param'al; column 4, line 14, for manetic read magnetic;

and that the said Letters Patent should be read with these correctionstherein that the some may conform to the record of the case in thePatent Office.

Signed and sealed this 25th day of April, A. D. 1950.

THOMAS F. MURPHY,

Assistant Oammiaaioner of Pam.

